Mass spectrometry



Feb. 17, 1 953 c, RQBINSON 2,629,055

MASS SPECTROMETRY Filed June 22, 1950 VEL OPE l0 AMPL/F/ER D/RE C T/ON OF MAGNET/C FIELD F/L/MENT FIG. 2. -COLLECTOR MAGNET POLE 3/ LECTRODE 34 POLE ELECTRON ELECTRON TARGET 26 GUN 20 PROPELL/NG ELECTRODE 25 INVENTOR. CHARLES F. ROBINSON AT TORNEY Patented Feb. 17, 1953 MASS SPECTROMETRY:

Charles F. Robinson, Pasadena, calif assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application June 22; 1950, .Serial No. 169,689

6 Claims. (01. 250-419) This invention is directed; to improvements inmass spectrometry, and more particularly relates'to mass spectrometry. wherein ions are separated on-the. basis of characteristic eriodicity of-motion in a magnetic field.

I-n mass spectrometrya sample to be analyzed is:ionized,as for example by'bombardment with an electron beam} and the various ions formed from the sample are separated-from each other on the basis of-some function of their mass -tocharge radio. In one type of operation this separationisbrought about by propelling the ionsunder the influence of suitable electrical potentials'from the point of their formation toward: atarget or collector electrode and in a path which carries them through a magnetic field. This method of mass spectrometry takes advantage-ofthefact that with a given initial kinetic energy, ions of differing mass-to-charge ratio will exhibit different radii of curvature under the influence of a magnetic field. A collector electrode is positioned to receive ions of a-givenmass-to-charge ratio. By varying the potential applied to propel the ions through the magnetic field or by varying the strength of the magnetic field through which the ions pass, ions of different mass-to-charge ratio can be selectively caused to impinge on and discharge at the collector electrode; The magnitude of the discharged current developed at the collector electrode by ions of a given mass-to-charge ratio'. is'afunction of. the partial pressure oi the parent particles in the gas sample}.

A more recent development in the field of mass spectrometry involves the segregation of ions" of differing mass-to cha'rge ratio on the basis of differences" in the resonant frequency of the ionsrthispmperty also being a function of the" mass-to-charge' ratio. This separation may be accomplished'by forming the ions in a space traversedby an alternating electrical field and. a magnetic field normal to the electrical field. The crossed fields' excite the ions to motion; thoseion'shaving'a resonant frequency corresponding to the frequency'of the alternating field" assuming a characteristic movement from the poiritof'origin'in the form of an expanding spiral'p'ath. A collector or target electrode disposed in the path of the resonant ionswill collect these ion's' while the non-resonant ions will travel in different" paths which theoretically will not carry them toth'e' target electrode.

By. altering the.- frequency of the alternating field, ions: of different' mass-to charge ratio in resonance with: the altered frequency will be caused. to renew the uniformlyv expanding spiral path which willcarry them to the collector electrode.' A mass spectrometer designed to operate on the foregoing, principles is described and illustrated' in co-pending application Serial No.

736L758; filed.-v March 24; 1947,1105, Harold WI Washburn;

A chargedp'article in'a magnetic field travels in a path ofra'diu's such that the centrifugal and magnetic forces are equal- Hence:

where:

m=the mass of theparticle v=velocity of travel r=radius of a curvature of; path q=particle charge, and

B=magnetic field strength The angular frequency W- in radians per second is' represented by the following" expression:

If a' particle of? mass and charge m and q, is placed in an AC field of frequency W0 such that the particle will ultimately attain any prea's signed radius and do so with energy 7 in accordance with the function Under certain types of electricalifield, 'as for example where the particle is. accelerated only across a narrow gap (as for example'in acyclotron) particles of mass 3m, 5m .'Will.also acquire energy at every half turn and will also attain any preassigned radius. Equation 5 shows that such particles will; have only /3 /s.. of the energy possessed" by" a resonant particle of mass in of low initial energy.

101155 of anomalous initial energies" although not in resonance with the alternating field may also reach the preassigned radius. Such ions pursue paths differing from the theoretical path and carrying them to the collector electrode. Such ions will reach the collector over a much wider range of frequencies than will ions of lower initial kinetic energy. The high kinetic energy of these ions in effect distorts their path of travel to spread over several mass units and thus reduces the resolution of the instrument.

I have now developed a method of operation whereby the harmonically related ions and the ions of anomalous initial kinetic energy will not interfere with the linear superposition of the resonant mass.

The invention contemplates, in mass spectrometry involving the formation of ions, separat on thereof on the basis of their resonant fre uencies by means of an alternating electrical field established across a space in which the ions are confined and a magnetic field normal to the electric field, and selectively collecting at a collector electrode those ions having a resonant frequency corresponding to the frequency of the alternating field, the improvement which comprises impressing on the collector electrode a positive potential of such magnitude as to repel therefrom ions having less than a predetermined minimum kinetic energy.

As explained above, ions having a high initial kinetic energy or non-resonant ions of harmonically related mass may achieve the collector electrode even though not in resonance with the alternating field. However, the resonant ions which are caused to pursue an ever expanding spiral path under the influence of the established field will approach the collector with appreciably greater energy than will those ions which reach the collector only by reason of initial energy distortion of their normal paths. It is possible, therefore, to impress on the collector a. potential of suflicient magnitude to repel all but the resonant ions.

The invention also contemplates apparatus for carrying out the foregoing method which comprises in a mass spectrometer having an analyzer chamber, means for ionizing a gas admitted to the chamber, means for establishing an alternating electrical field across the space normal to the electrical field, means for establishing a magnetic field across the space normal to the electric field, and a collector electrode disposed in said space in the path of ions having a resonant frequency corresponding to the frequency of the alternating field, the improvement comprising means for impressing on the collector electrode a positive potential, and means for varying said positive potential to repel from the collector electrode ions having less than a predetermined energy.

The invention will be more clearly understood from the following detailed description thereof taken in conjunction with the accompanying drawing in which:

Fig. 1 is a perspective diagrammatic view of one form of the apparatus; and

Fig. 2 is a sectional elevation taken on the line 22 of Fig. 1.

Referring to the drawing, the embodiment of the invention there shown comprises an envelope In with a rectangular conductive box or shell I2 disposed therein. A pair of plate electrodes I4, I5 are mounted adjacent to and substantially enclosing the open ends of the-box I2 within the envelope and are insulated from the box as by the illustrated small gap.

A gas inlet tube I6 is carried through a wall of the envelope and opens into an inlet chamber I8 conveniently mounted on a side I2A of the box I2. An electron gun 20 and a propelling electrode 22 are mounted adjacent an opposite side I2B of the box to direct an electron beam axially through the box, the beam passing through apertures 23, 24 in the box Walls I2B, I2A respectively, to impinge on an electron target 26 mounted in the inlet chamber I8. A propelling electrode 25 is mounted in the chamber I 8 and has an} aperture 25A. Any ions formed in the inlet chamber are propelled into the shell by a pro pelling potential established between the chamher and electrode 25.

The arrangement is such that an electron beam generated at the gun 20 is directed through electrode 22 and through the shell I2 along a central axis thereof and roughly midway between electrodes I4, I5 to the target electrode 26. The gas molecules introduced through the inlet tube I 6 migrate into the box I2 by diffusion and are ionized therein. Since the electron beam travels axially through the shell, gas contained therein will be ionized as it is impacted by this beam, and hence all ions originate along the axis defined by the electron beam.

Magnetic pole pieces 30, 3| are mounted outside the envelope I0 and are oriented to develop a magnetic field in the box parallel to the electron beam and normal to the alternating field developed between the electrodes I 4, I5. The magnetic pole pieces may be mounted within the envelope or may form a part of the envelope with equal facility, as will be obvious to anyone familiar with the art. A collector electrode 34 extends into the space defined by the box I2 conveniently through one of the electrodes I4, f5 and with its collecting face lying parallel to the electron beam.

An amplifier 36 is connected to the collector electrode and to a bias battery 3! through a slidewire 38 and adjustable tap 39. A high frequency oscillator 49 is connected across the electrodes I4, I5 through a transformer 4| with the electrodes being connected to opposite ends of the secondary winding 4IA of the transformer. The oscillator supplies a high frequency alternating voltage to the electrodes so that a high frequency A. C. field is developed across the space defined by the shell I2. The negative side of the bias battery is connected to the midpoint of transformer secondary MA and to the case I 2. The collector is thus always at a positive potential with respect to the electrodes I4, I5 and case I2. In this manner the minimum kinetic energy required to drive ions through the repelling field to the collector can be varied as desired.

A filament power supply 46 is connected by appropriate leads to the electron gun 20, inlet chamber I8, electrode 22, electron target 26 and electrode 25 to supply the necessary voltages to these various elements.

The operation of the instrument is as follows. With the envelope I0 evacuated, as is standard practice in mass spectrometry, a gas sample is introduced through inlet I6 and is ionized within the shell I2 whenever molecules of the same intersect the electron beam. Under the influence of the high frequency alternating field and the magnetic field normal thereto, the ions are set in motion within the confines of the shell I2 with the ions of varying mass-to-charge ratio follow- 8 ing different paths of travel. Iheresonent ions, that is, the ions of a givenmass-to-charge ratio which have a resonant frequency corresponding to the frequency of alternationoi the A. C. field, travel in a uniformly expanding-spiral path illus- 5 trated at 54 in Fig. 2. With the shell l2 at a slight positive potential with respect to the electrodes l4, 15, the ions are reached from the walls IZA and 123 of the shell. Thus ions formed at.

the midpoint of the electron'beam, 'i. e. midway l 7 between walls I'ZA, IZB, will -remain in a plane transverse to the beam and intersecting themidpoint, while ions formed elsewhere along the beam will oscillate acro s the midpoint. When the radius of travel of the-resonant ions is such asl= 5 to reach the collector electrode 34, these ions willg be discharged. The hon-eresonant ions which do;

not attain a radius of travel equal to the spac-Ig ing between the collector electrode and the axis of origin along the electron beam, travel back and forth within the shell, as for example in spira paths which expand to a maximum and collapse back to the origin. These non-resonant ions, therefore, accumulate in the box. It may be desirable to provide for periodic removal of t iv 2 accumulated ions, but such provision is outside the scope of this invention.

As indicated above, certain non-resonant ions Y i will have amass harmonically related to the mas o of resonant ions and these harmonic ions In reach the collector electrode. Also some such non-resonant ions are possessed of anomalous initial energy which will result in an anomalous center of curvature of their spiral paths so that they may reach the collector electrode before col:- lapsing back to their origin. In either of th e cases, erroneous discharge current values will: e, obtained. In the illustrated apparatus, this prq lem is overcome by biasing the collector electrqde 1 so that it is always slightly positive with respect to the sh m I2 and the electrodes l4, l5. 4 positive bias may be adjusted by means of slidewire 38 so that any ions having less kinetic gnergy than the resonant ions will be repelled [ii I the collector electrode. It is possible to prov e that regardless of the magnitude of initial kinct'c energy of the non-resonant ions they will no they approach the collector electrode, have kinetic energies of the order of magnitude of the resonant ions to which kinetic energy is impartetg by the driving field. If the bias of the collector elec 5 trode is properly adjusted, only those ions which are in resonance with the fundamental frequency I of the A. C. field will be capable of striking and discharging at the collector electrode while e non-resonant harmonically related ions or the. non-resonant or resonant ions of anomalous nitial energies will be repelled. 1

As illustrated, the output of the collector e ectrode will-"be in the nature of a pulsating rent, thefrequency of which will depend rip 'n the frequency of alternation of the alternaiting electric field. This pulse type output current Shay be amplified inan A. C. amplifier having guitable frequency response or in a D. C. amplifier since the frequency is so high as to appear-as a steady current in such amplifier.

If an A. C. amplifier is used, spurious signals a may result from electrostatic coupling betfi een collector electrode 34 and electrode 14, an A. C. signal being induced in collector 34 by the C. accelerating field. This can be neutralizedby placing a small capacity such as trimmer" capacitor 50 between collector electrode 34;;and electrode 1 [5. Such a capacitor, properly adjusted, will maintain the collector 31 at zero po- 75.

. 'tent iel for R. F. signals. HA. 0. ampliflcationat lower frequency is desired, the discharge current may be pulsed by other means, as for example modulation of the electron beam. The present invention is independent of any special provision for pulsing the discharge current at a frequency other than the frequency of the A. C. field or for removing non-resonant ions from the analyzer chamber.

Iclaim:

I. In a mass spectrometer having an analyzer chamber, means for ionizing a gas admitted to the chamber, means for establishing an alternating electrical field across the space defined by the chamber, means for producing a magnetic field across the space normal to the electrical field, and a collector electrode disposed in said space in the path of ions having a resonant frequency corresponding to the frequency of the alternating field, the improvement comprising means for biasing said collector electrode, and means for varying said bias to repel from the collector electrode ions having less than a predetermined minimum kinetic energy. 5 2. In a mass spectrometer having an analyzer chamber, means for ionizing a gas admitted to the chamber, means for establishing an alternating electrical field across the space defined by the chamber, means for producing a magnetic field across the space normal to the electrical field, and a collector electrode disposed in said space in the path of ions having a resonant frequency corresponding to the frequency of the alternating field, the improvement comprising means for maintaining said collector electrode at a positive potential with respect to the chamber andmeans for varying said potential to repel from the collector electrode ions having less than a predetermined minimum kinetic energy.

3. ma mass spectrometer having an analyzer chamber, means for ionizing a gas admitted to the chamber, means for establishing an alternating electrical field across the spaced defined by the chamber, means for producing a magnetic field across the space normal to the electrical field, and a collector electrode disposed in said space in the path of ions having a resonant frequency corresponding to the frequency of the alternating field, the improvement comprising a source: of D. C. voltage, means connecting the collector electrode to the positive side of said source, and mean for connecting the chamber to the negative side of said source.

4. In a mass spectrometer having an analyzer chamber, means for ionizing a gas admitted to the chamber, means for establishing an alternating electrical field across the space defined by the chamber, means for producing a magnetic' field across the space normal to the electrical field, and a collector electrode disposed in said space in the path of ions having a resonant frequency corresponding to the frequency of the alternating field, the improvement comprising a source of D. C. voltage, means connecting the collector electrode to the positive side of said source, means for connecting the chamber to the negative side of said source, and means for varying the potential impressed across the collector electrode and chamber by said source.

5. In a mass spectrometer having an analyzer her, a pair'of electrodes disposed at opposlteends of the chamber and insulated from the chamber walls, means for impressing a high frequency alternating potential between said electrodes to .establish an A. C. field in said chamber transverse to the electron beam, means for producing a magnetic field across the chamber normal to the A. C. field and parallel to the electron beam and a collector electrode disposed in said chamber in the path of ions having a resonant frequency corresponding to the frequency of the alternating field, the improvement comprising an -A. C. amplifier connected to receive the discharge connected between the collector electrode and one of said pair of electrodes.

6. Ina mass spectrometer having an analyzer chamber, means for ionizing ga admitted to the chamber, means for establishing an alternating electrical field across the space defined by the chamber, means for producing a magnetic field .acrossthe Space normal to the electrical field,

and an electrode disposed in the path of ions having a resonant, frequency corresponding to the frequency of the alternating field for collecting such ions, .theimprovement comprising means for biasing said electrode to repel therefrom ions having-less than a predetermined kinetic energy.

CHARLES F. ROBINSON.

No references ci e 

